The ecteinascidins, a family of tetrahydroisoquinoline alkaloids isolated from the Caribbean tunicate Ecteinascidia turbinate, (Wright, A. E. et al. J. Org. Chem. 1990, 55, 4508-4512; Rinehart, K. L. et al. J. Org. Chem. 1990, 55, 4512-4515; Rinehart, K. L. et al. J. Org. Chem. 1991, 56, 1676; Sakai, R. et al. Proc. Nat. Acad. Sci. U.S.A. 1992, 89, 11456-11460; Sakai, R. et al. J. Am. Chem. Soc. 1996, 118, 9017-9023; Suwanborirux, K. et al. J Nat. Prod. 2002, 65, 935-937) possess potent cytotoxic activity against a variety of tumor cell lines in vitro and against several rodent tumors and human tumor xenografts in vivo (Rinehart, K. L. Med. Drug. Rev. 2000, 1-27). One of its members, ecteinascidin 743 (Et 743, 1a, below) is currently in phase II/III clinical trials in Europe and the United States for ovarian, endometrium, breast cancer and several types of sarcoma. It showed particularly high activity in cases of advanced sarcoma that had relapsed or were resistant to conventional therapy. Et 743 (commercial name: Yondelis®) has been granted Orphan Drug Designation by the US Food and Drug Administration (FDA, 2005) and European Commission (2003) for the treatment of ovarian cancer. The antiproliferative activity of Et 743 is greater than that of taxol, camptothecin, adriamycin, mitomycin C, cisplatin, bleomycin and etoposide by 1-3 orders of magnitude. Et 743 binds to the minor groove of the DNA by way of three hydrogen bond contacts between the A- and E-ring of Et 743 and the three base pairs recognition sequence, the most critical being the interaction of the E-subunit with the base located 3′ to the modification site. In addition, through intramolecular acid-catalyzed dehydration of the carbinolamine moiety, Et 743 forms a covalent bond with the exocyclic 2-amino group of guanine (Pommier, Y. et al. Biochemistry 1996, 35, 13303-13309; Moore, B. M. et al. Am. Chem. Soc. 1998, 120, 2490-2491). It was demonstrated that the formation of Et 743/DNA complex is reversible under non-denaturing conditions and that Et 743 can migrate from the non-favored bonding sequence (e.g., 5′-AGT) to the favored DNA target site (e.g., 5′-AGC), leading to the observed site-specificity (Zewail-Foote, M. et al. J. Am. Chem. Soc. 2001, 123, 6485-6495). In the Et 743/DNA adduct, the double helix bends toward the major groove and the third domain (ring F-G) of Et 743 positions itself outside the complex, making it available to interact with proteins and at the same time disrupting DNA-protein binding (Moore, B. M. et al. J. Am. Chem. Soc. 1997, 119, 5475-5476; Zewail-Foote, et al. J. Med. Chem. 1999, 42, 2493-2497; Garcia-Nieto, R. et al. J. Med. Chem. 2000, 43, 4367-4369; Garcia-Nieto, R. et al. J. Am. Chem. Soc. 2000, 122, 7172-7182; Seaman, F. C. et al. J. Am. Chem. Soc. 1998, 120, 13028-13041 Takebyashi, Y. et al. Nature Med. 2001, 7, 961-966; Zewail-Foote, M. et al. Chem. Biol. 2001, 8, 1033-1049). Although the F-G subunit has little contact with the minor groove of DNA, its presence is of utmost importance for the antitumor activity of Et 743. Indeed, it has been shown that modifying the F-G subunit changes the drug's ability to inhibit cell division. For example, Et 736 (1c) with a tetrahydro-β-carboline residue instead of a tetrahydroisoquinoline at the F-G part has different bioactivity profile relative to Et 743. It is only slightly active vs M5076 ovarian sarcoma and an MX-1 human mammary carcinoma xenograft, but shows a higher level of activity in vivo in mice against P388 leukemia (Sakai, R. et al. Proc. Nat. Acad. Sci. U.S.A. 1992, 89, 11456-11460; Jin, S. et al. Proc. Nat. Acad. Sci. U.S.A. 2000, 97, 6775-6779; Minuzzo, M. et al. Proc. Nat. Acad. Sci. U.S.A. 2000, 97, 6780-6784). The Et 637 (1e) and Et 594 (1f), lacking the F-G subunit, are generally 10-50 times less active than Et 743 against MEL 28 and CV-A cell lines (Sakai, R. et al. J. Am. Chem. Soc. 1996, 118, 9017-9023).
Structurally, Et 743 is constituted of three tetrahydroisoquinoline systems interconnected via two bridged ring systems. Specifically, ring A-B and ring D-E are fused together producing an additional 6-membered ring (ring C) and a labile carbinolamine functional group that serves to alkylate the DNA. In addition, ring A-B is linked to the third tetrahydroisoquinoline (F-G) by a 10-membered lactone having a 1,4-bridged benzylic sulfide linkage. Overall seven stereocenters and eight rings are found in Et-743. Et 743 is structurally related to the saframycin class of antibiotics (Arai, T. et al. J. Antibiot. 1977, 30, 1015-1018; Arai, T. et al. The Alkaloids Brossi, A. Ed.; Academic Press: New York, 1983, V 21, pp 55-100), the noticeable difference being the higher oxidation state of C-4 carbon in Et 743 than in saframycin (2, 3). The same difference can be recognized in two other structurally related natural products, naphthyridinomycin (4) (Itoh, J. et al. Antibiot. 1982, 35, 642-644) and lemonomycin (5, Structures of ecteinascidin 743 and related natural products, provided below.) (He, H. et al. Tetrahedron Lett. 2000, 41, 2067-2071).
Due to the extremely low natural availability (1 gram from 1 ton of tunicate), the drug supply is becoming a key issue. PharmaMar has tried growing the sea squirt on underwater farms (300 tonnes) in Puerto Rico and Spain but only with limited success. To obtain enough amount of drug for cancer treatment, a simpler and more efficient process was thus needed. Total synthesis or hemisynthesis from simpler natural product became an important alternative and, in this particular case probably the only available alternative (Scott, J. D. et al. Chem. Rev. 2002, 102, 1669-1730).
To date, two total syntheses have been accomplished by Corey (J. Am. Chem. Soc. 1996, 118, 9202-9203; Org. Lett. 2000, 2, 993-996) and Fukuyama (T. Synlett 1999, 1103-1105; J. Am. Chem. Soc. 2002, 124, 6552-6554) respectively. A semi synthesis from cyanosaframycin B (3) has been developed by Cuevas, Manzanares and co-workers at PharmaMar (Org. Lett. 2000, 2, 2545-2548; J. Org. Chem. 2003, 68, 8859-8866). In addition, other synthetic approaches have been reported from a number of research groups, including that of Kubo (J. Chem. Soc. Perkin Trans 1 1997, 53-69; Heterocycles 1999, 51, 9-12; A. Chem. Pharm. Bull. 2000, 48, 1549-1557), Danishefsky (Tetrahedron Lett. 2000, 41, 2039-2042; Tetrahedron Lett. 2000, 41, 2043-2046; Org. Lett. 2002, 4, 43-46, Chem. Int. Ed. 2006, 45, 1754-1759), Williams (Tetrahedron Lett. 2001, 42, 543-546; Tetrahedron Lett. 2003, 44, 4635-4639; Org. Lett. 2003, 5, 2095-2098), Magnus (Org. Lett. 2003, 5, 2181-2184) and Liu (Tetrahedron Lett. 2003, 44, 7091-7094). A simpler synthetic analog of Et 743 named phthalascidin (Pt-650) that displayed virtually the same biological activities as the natural product has been discovered by Corey and Schreiber (Proc. Natl. Acad. Sci. U.S.A. 1996, 96, 3496-3501).
While both Corey and Fukuyama's syntheses are landmark achievement in organic synthesis, they are difficult to be applied into a large-scale production.
An alternative synthetic approach has been investigated and preliminary result has been published dealing with the synthesis of pentacyclic compound of Et 743 (De Paolis, M. et al. Chem. Soc. Chem. Commun. 2003, 2896-2897; De Paolis, M. et al. Synlett 2004, 729-731; Chen, X. et al. J. Org. Chem. 2005, 70, 4397-4408).